Abstract In the first part of the present study, a thermal-hydraulic subchannel code hereafter called ‘SUBTHAC’ is developed to evaluate the enhancement effects of nanoparticles in core heat transfer. The first version of SUBTHAC (V1.0) can analyze the steady state flow of coolant with Al2O3, TiO2 or CuO as nanoparticles (other types of nanoparticles can be added by the user). Different output profiles can be selected such as fluid temperature, pressure and velocity for each subchannel, clad outside temperature for each fuel rod, axial and lateral mass flow, etc. SUBTHAC uses a dedicated algorithm to solve the subchannel equations and, unlike many other codes, allows for thermophysical parameters of nanoparticles to be a function of the temperature, leading to improvement the accuracy of results. Results computed by SUBTHAC for base fluid (pure water) are validated against those obtained by COBRA-EN code. In the next step, with the aim of validating the capability of nanofluid analysis of SUBTHAC code, its nanofluids results have been validated against reference CFD simulations. After the validation, comprehensive numerical comparisons are conducted to assess the enhancement of thermal-hydraulic parameters by using nanofluids. It is shown that, among Al2O3, TiO2 and CuO nanofluids with volumetric concentration in the range of 1–5%, TiO2-3% and CuO-3% are the best choices to increase fluid outlet temperature and decrease clad temperature, respectively. Using nanofluids with a concentration higher than 3% volumetric is not justifiable as the core pressure drop increases up to more than 20%. In the second part of the manuscript, some relevant remarks are put forward on the assignment of boundary conditions (BC, i.e. inlet velocity/inlet mass flux/inlet Reynolds number) and the adoption of reliable values for specific heat capacity of nanoparticles in operational temperature of NPPs. The effects of using the above boundary conditions and incorrect values of the specific heat (as adopted in the literature so far) are depicted by presenting some profiles of coolant and clad temperature. Selecting different BCs and incorrect values of specific heat for nanoparticles can jeopardize the results of calculations.

中文翻译：

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纳米粒子的边界条件和热物理特性在纳米流体作为核电厂冷却剂的应用中的应用；数值研究

摘要 在本研究的第一部分中，开发了一种称为“SUBTHAC”的热工水力子通道代码，用于评估纳米颗粒在核心传热中的增强效果。第一个版本的 SUBTHAC (V1.0) 可以分析冷却剂的稳态流动，以 Al2O3、TiO2 或 CuO 作为纳米颗粒（用户可以添加其他类型的纳米颗粒）。可以选择不同的输出曲线，例如每个子通道的流体温度、压力和速度、每个燃料棒的包层外部温度、轴向和横向质量流等。 SUBTHAC 使用专用算法来求解子通道方程，并且与许多其他代码不同，允许纳米粒子的热物理参数是温度的函数，从而提高结果的准确性。SUBTHAC 为基础流体（纯水）计算的结果与 COBRA-EN 代码获得的结果进行了验证。在下一步中，为了验证 SUBTHAC 代码的纳米流体分析能力，其纳米流体结果已针对参考 CFD 模拟进行了验证。验证后，进行全面的数值比较，以评估使用纳米流体对热工水力参数的增强。结果表明，在体积浓度为1%~5%的Al2O3、TiO2和CuO纳米流体中，TiO2-3%和CuO-3%分别是提高流体出口温度和降低包覆温度的最佳选择。使用浓度高于 3% 体积的纳米流体是不合理的，因为核心压降增加到 20% 以上。在手稿的第二部分，对边界条件（BC，即入口速度/入口质量通量/入口雷诺数）的分配和在核电厂运行温度下采用可靠的纳米颗粒比热容值提出了一些相关评论。使用上述边界条件和不正确的比热值（迄今为止在文献中采用的值）的影响通过提供冷却剂和包层温度的一些曲线来描述。为纳米粒子选择不同的 BC 和不正确的比热值会危及计算结果。使用上述边界条件和不正确的比热值（迄今为止在文献中采用的值）的影响通过提供冷却剂和包层温度的一些曲线来描述。为纳米粒子选择不同的 BC 和不正确的比热值会危及计算结果。使用上述边界条件和不正确的比热值（迄今为止在文献中采用的值）的影响通过提供冷却剂和包层温度的一些曲线来描述。为纳米粒子选择不同的 BC 和不正确的比热值会危及计算结果。